1. Field of the Invention
The present invention relates to an image forming apparatus such copiers, printers and the like using electrophotographic methods.
2. Description of the Related Art
In image forming apparatus such as copiers, printers and the like using electrophotographic methods, typically charge a photosensitive drum by means of a charger, perform optical image exposure of said charged region so as to form an electrostatic latent image on the surface of said drum, said latent image then being developed so as to produce a visible image which is then transferred to a transfer member and fixed thereon.
In recent years, various apparatus have been proposed which omit a cleaning device in conjunction with demand for inexpensive and more compact apparatus.
For example, U.S. Pat. No. 5,148,219 discloses a so-called cleanerless image forming apparatus which combines a cleaning device with the developing device. Furthermore, U.S. Pat. No. 4,469,435 discloses a cleanerless image forming apparatus which combines a cleaning device with a charger having a contact member which makes contact with a photosensitive member. In this instance, the contact-type charger accomplishes cleaning by electrostatically adhering residual developer to the charging member.
The aforesaid cleanerless image forming apparatus, however, pose the following disadvantages.
In image forming apparatus provided with a cleaning device, the photosensitive member is shaved simultaneously with the removal of the residual developer from the surface of the photosensitive member by means of a cleaning member such as a blade, brush or the like, but when such a cleaning device is not provided, ozone accumulation causes deterioration of the surface of the photosensitive member and results in toner filming because a suitable shaving member is not provided for the photosensitive member. Ozone-induced deterioration is a phenomenon wherein ozone generated by a charger, transfer device or the like corrodes the surface of the photosensitive member. Toner filming is a phenomenon wherein a thin film of toner, i.e., developer, adheres to the entire surface of the photosensitive member.
When ozone-induced deterioration or toner filming occurs and electrostatic latent images are formed, and particularly when latent images are formed under conditions of high temperature and high humidity, there is a horizontal drift of the charge on the surface of the photosensitive member which disrupts said latent image, causing so-called image drift. The occurrence of image drift, for example, in the case of reversal development wherein toner is adhered to the optically exposed regions in a halftone dot pattern image such as shown in FIG. 6, the surface potential of the photosensitive member (e.g., -600 V) is locally reduced (e.g., to -50 V) in dot-like spots d1 by optical exposure, as shown in FIG. 7A, at which locations the dot to be developed in black or other color is eliminated due to charge drift, as shown in FIG. 7B.